A light-sensitive image forming material has a layer comprising a light-sensitive resin which is capable of forming an image by imagewise exposure. The light-sensitive resin comes to be soluble or insoluble in a solvent accompanying to its molecular structure change caused by exposure of light. The formation of an image is performed by taking advantage of an above phenomenon of the light-sensitive resin. The light-sensitive image forming material having such light-sensitive resin layer is utilized in the fields of printing plates such as a PS (Presensitized) lithograph and a resin relief plate, photoresists such as a film type photoresist, color proofs in prepress, and lithfilm for dot to dot work.
For example, a light-sensitive image forming material employable for a proof to confirm hue and tone of a finished print has a structure comprising a support (may be referred to as tentative support hereinafter) and a peel layer comprising an organic polymer and a light-sensitive resin layer provided thereon in order. In the case of using the light-sensitive image forming material for an image forming material (light-sensitive transferring sheet) in the surprint method (which is one of color proofing methods), the material is imagewise exposed to light to form an image on the peel layer through developing and then the resultant separation image is transferred to a desired support to prepare a color proofing sheet having the separation image.
Such color proofing sheet is used to confirm hue and tone of a finished print as mentioned above, so that a separation image formed on a light-sensitive image forming material is desired to show a distinct hue.
To form a color image on a light-sensitive image forming material, a colorant material in the light-sensitive resin layer generally uses an organic pigment which is excellent in various characteristics such as richness of hue. In more detail, a color image is obtained by applying a coating solution prepared through mixing the organic pigment and a light-sensitive resin to a peel layer provided on a support to prepare a light-sensitive image forming material, and imagewise exposing the material to light and then removing a soluble portion of the resin layer by developing by the use of a developing solution.
For the purpose of obtaining a color image which is excellent in the above characteristics, Japanese Provisional Patent Publication Nos. 62 (1987)-67529 and 63 (1988)-74052 propose use of a lake pigment (containing chelate pigment) as a colorant material. The lake pigment has an anionic group such as a carboxylic acid group or a sulfonic acid group as a substituent on its molecular, which forms lake with a mono- or di-valent metal ion to produce its metal salt. The lake pigment is generally apt to be insoluble in various coating solutions. Accordingly, in the case of applying a coating solution for the light-sensitive resin layer to other layer (particularly a peel layer), the pigment scarcely dyes the peel layer so that an image having a distinct hue is deemed to be obtained.
The study of the present inventors, however, have revealed that even use of the lake pigment does not give occasionally a distinct image. The inventors have further studied and found that the lake pigment contains in part a pigment which have not formed lake with the metal ion and such pigment is apt to penetrate into the peel layer in the course of provision of a light-sensitive resin layer or development of the resin layer. As a result, the peel layer is dyed by the pigment, i.e., fog of a coloring material is apt to occur on the peel layer.